Antennas are indispensable core devices in wireless communications [1]. To meet requirements in different application scenarios, various antennas have been proposed [[2], [3], [4]]. Low-gain antennas with omnidirectional radiation characteristics, low cost, and light weight are often used in mobile communications, such as patch antennas [5] and slot antennas [6]. However, for long-distance communications such as satellite communications and deep space exploration, low-profile antennas with high gain and low sidelobe levels (SLLs) play an important role. Traditional parabolic antennas [7] and lens antennas [8] can achieve beamforming by changing the surface curvature radius, but they have the disadvantages of large volume, and high profile. The microstrip array antenna has been widely valued, owing to the planar structure, mature processing technology, and flexible feeding. Various microstrip array antennas have been proposed, such as microstrip patch array antennas [9,10], microstrip dipole array antennas [11,12], microstrip leaky wave array antennas [13], and microstrip traveling-wave array antennas [14]. The reflectarray antenna [15] combines the advantages of the traditional parabolic antenna and the planar microstrip array antenna, in which the space feed avoids the need for a complicated feeding network. The transmitarray antenna [16] is developed to locate the receiving and the re-radiation signals on different sides of the antenna, and thus the blocking of the re-radiation wave by the feed source is avoided. Many functions have been realized by the transmitarray antennas, including dual-band manipulation [12,17], beam scanning [18,19], ultra-wide bandwidth [11], multi-beams [20,21] and so on. However, these works mainly focus on the frequency bands lower than that of terahertz (THz) waves.

With the increasing requirement of data transmission rate in many application fields, the current wireless communication frequency bands are becoming more and more crowded. The THz band (0.1−10 THz) has received more and more attention owing to its large bandwidth and high transmission rate [22]. However, the propagation loss of THz waves in atmosphere is very high [23]. Therefore, THz antennas with high gain and low sidelobes are of great significance in THz long-distance communications. A microstrip patch antenna with a resonant frequency higher than 115 GHz is proposed to achieve a peak gain of 18.47 dBi and low SLLs of −29.6 dB by configuring the size of each element in the antenna array and the value of the feed [24]. Aimed at the sparse array antenna in the THz band, a back-projection (BP) algorithm is used to reconstruct the target, and a sidelobe suppression method based on the coherence factor is proposed to improve the image quality. The simulation results show that the suppressed sidelobe is 29 dB lower than the main lobe [25]. A THz transmitarray antenna is proposed to realize a high-gain of ∼30 dBi and an optional polarized state [26]. To realize beamforming, an ultra-massive MIMO antenna is designed and simulated, which can achieve highly directional narrow beams in the working frequency band of 0.06–10 THz [27]. Moreover, graphene is applied to the miniaturized THz antenna, which has the advantage of deep subwavelength scale [[28], [29], [30]]. However, it is still a challenge to simultaneously achieve high-gain and low-sidelobe in the THz range, especially at dual THz bands.

In this study, a metallic waveguide transmitarray antenna (MWTA) is proposed to achieve simultaneous beam shaping of two orthogonally polarized dual-band THz waves. The phase delays of the two waves can be independently controlled in the range of [0, 2π] by adjusting the dimension of each unit cell in the array. Based on the grating diffraction principle, the high-order diffraction maxima can be suppressed by tuning the array structure and the corresponding phase distribution, and thus the high-gain and low-sidelobe beams with different beam directions can be simultaneously realized at dual bands. To verify the feasibility of the method, a MWTA with a diameter of 48 mm is designed and simulated. It can independently control the phases of x- and y-polarized waves with frequencies of 0.14 and 0.0972 THz, respectively, allowing the dual-band beam control at the same time. The corresponding peak gains are 32 and 28 dBi. The SLLs are both 21 dB lower than the main lobes, and the bandwidth are 46 and 17 GHz, respectively. As a proof of concept, a prototype is fabricated and the measured results are highly consistent with the simulations.